Water
PollutionWater pollution
is a large set of adverse effects upon water bodies such as lakes, rivers, oceans,
and groundwater caused by human
activities.
Although natural phenomena such as volcanoes, algae blooms, storms, and earthquakes also cause major
changes in water quality and the ecological status of water, these are not
deemed to be
pollution. Water is only called polluted when it is not able to be
used for what one wants it to be used for. Water pollution has many causes and
characteristics. Increases in nutrient loading may lead to eutrophication. Organic wastes
such as sewage impose high oxygen demands on the receiving water leading to oxygen
depletion with potentially severe impacts on the whole eco-system. Industries
discharge a variety of pollutants in their wastewater including heavy metals, resin pellets,
organic toxins, oils, nutrients, and solids. Discharges can also have thermal
effects, especially those from power stations, and these too reduce the
available oxygen. Silt-bearing
runoff from many activities including construction sites, deforestation and agriculture can inhibit the
penetration of sunlight through the water column, restricting photosynthesis and causing
blanketing of the lake or river bed, in turn damaging ecological systems.
Pollutants in water include a wide spectrum
of chemicals, pathogens, and physical
chemistry or sensory changes. Many of the chemical substances are toxic. Pathogens can produce waterborne diseases in either human or animal hosts. Alteration of
water's physical chemistry include acidity, conductivity, temperature, and
eutrophication. Eutrophication
is the fertilisation
of surface water by nutrients that were previously scarce. Even many of the municipal water supplies in
developed countries can present health risks. Water pollution is a major
problem in the global context. It has been suggested that it is the leading
worldwide cause of deaths and diseases, and that it accounts for the deaths of
more than 14,000 people daily.
Sources
of water pollution
Some of the principal sources of
water pollution are:
Geology of aquifers from which groundwater is
abstracted
Industrial discharge of chemical wastes
and byproducts
Discharge of poorly-treated or untreated sewage Surface runoff
containing pesticides
or fertilizers
Slash and burn
farming practice, which is often an element within shifting
cultivation agricultural systems Surface runoff
containing spilled petroleum
products Surface runoff from construction sites, farms, or paved
and other impervious surfaces e.g. silt Discharge of contaminated and/or heated water used for
industrial processes Acid rain caused by industrial discharge
of sulphur dioxide (by burning high-sulphur
fossil fuels)
Excess nutrients are added (eutrophication) by
runoff containing detergents
or fertilizers
Underground
storage tank leakage, leading to soil contamination, and hence aquifer contamination Inappropriate disposal of various solid wastes and, on a localized scale, littering Oil spills
effect
Most
water pollutants are eventually carried by the rivers into the oceans. In some
areas of the world the influence can be traced hundred miles from the mouth by
studies using hydrology transport models. Advanced computer models such as SWMM or the DSSAM Model have been used in
many locations worldwide to examine the fate of pollutants in aquatic systems.
Indicator filter feeding species such as copepods have also been used to
study pollutant fates in the New York Bight, for example.
The highest toxin loads are not directly at
the mouth of the Hudson River,
but 100 kilometers south, since several days are required for incorporation
into planktonic tissue. The Hudson
discharge flows south along the coast due to coriolis force. Further south
then areareas of oxygen depletion, caused by chemicals using up oxygen and by algae blooms, caused by excess nutrients from algal cell death
and decomposition. Fish and shellfish kills have been
reported, because toxins climb the foodchain after small fish consume copepods, then large fish eat
smaller fish, etc. Each successive step up the food chain causes a stepwise
concentration of pollutants such as heavy metals (e.g. mercury) and persistent organic pollutants such as DDT. This is known as biomagnification which is
occasionally used interchangeably with bioaccumulation.
The big gyres in the oceans trap floating
plastic debris. The North Pacific Gyre for example has collected the so-called Great Pacific Garbage Patch that is
now estimated at two times the size of Texas. Many of these long-lasting pieces
wind up in the stomachs of marine birds and animals. This results in
obstruction of digestive pathways which leads to reduced appetite or even
starvation.
Many chemicals undergo reactive decay or chemially change especially over long periods of
time in groundwater
reservoirs. A noteworthy class of such chemicals are the chlorinated hydrocarbons such as trichloroethylene (used in
industrial metal degreasing and electronics manufacturing) and tetrachloroethylene used in the dry cleaning industry (note latest
advances in liquid carbon dioxide in dry cleaning that avoids all use of
chemicals). Both of these chemicals, which are carcinogens themselves, undergo
partial decomposition reactions, leading to new hazardous chemicals (including
dichloroethylene and vinyl chloride).